When you use a Microsoft Skype for Business client that's authenticated by the TLS-DSK authentication or modern authentication (MA), you may find that you can't sign in as an external user. This issue occurs when TLS1.2 is enabled for the Skype for Business client.
To fix this issue, install one of the following updates:
For Skype for Business 2016, install the April 3, 2018, update (KB4018323).
For Skype for Business 2015 (Lync 2013), install the April 3, 2018, update (KB4018334).
Note Lync 2013 was upgraded to Skype for Business in April 2015.